Several processes have been used commercially or have been proposed to produce an oxygen-enriched air product by fractionation of air into its constituent components.
In U.S. Pat. No. 3,210,951, a fractionation cycle employing first and second fractionating zones operating under different pressures and including two reboiler/condensers is disclosed. Both of the reboiler/condensers are interconnected with the stages of fractionation in such a manner as to effect the required reboil and reflux production with minimum pressure differential between the stages of rectification and also decrease the irreversibility of the overall fractionation process thereby obtaining the desired separation with the high pressure stage operating under substantially reduced pressure.
In U.S. Pat. No. 3,277,655, an improvement to the fractionation process taught in U.S. Pat. No. 3,210,951 is disclosed. In this process, the heat exchange occurring in one of the two reboiler/condensers between the bottoms liquid from the lower pressure column and the gaseous material from the high pressure column results in complete liquefaction of the liquid from the low pressure column thereby satisfying the reboiler requirements of the low pressure column. Additionally, when the liquefied gaseous material from the high pressure column is introduced into the lower pressure column it improves the reflux ratio in the upper portion of the low pressure column which increases the separation efficiency and makes it possible to lower the pressure of the gaseous mixture entering the cycle.
In U.S. Pat. No. 3,327,489, another improvmeent to U.S. Pat. No. 3,210,951 to lower the pressure in the high pressure fractionator is disclosed. In the process, the pressure reduction is obtained along with the associated power reduction by establishing a heat exchange between gaseous material, which may comprise the feed mixture, and a liquid component collecting in the bottom of the low pressure fractionator, with the liquid component being under different pressure.
In U.S. Pat. No. 3,754,406, a process is disclosed for the production of low purity oxygen, in which a low pressure stream of incoming air is cooled against outgoing gas streams and fed into a high pressure distillation column. A high pressure stream of incoming air is cooled against outgoing gas stream, partially condensed against boiling oxygen product in a product vaporizer, and separated into gas and liquid streams. The liquid stream being subcooled and expanded into a low pressure fractionating column. The gas stream is reheated and expanded to provide process refrigeration and is introduced into the low pressure fractionating column. Crude liquid oxygen from the bottom of the high pressure column is cooled and introduced into the low pressure column after being used to liquefy some of the nitrogen from the high pressure column in an external reboiler condenser. Liquid oxygen produced from the low pressure column is pumped to a higher pressure before being passed to the subcooler and the product vaporizer. The remainder of the high pressure nitrogen is liquefied in a second external reboiler/condenser and is used as reflux for the two columns. A waste nitrogen stream is removed from the low pressure column.